The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Brazing is a technique of assembling two materials which is based on the differentials of the thermal expansion coefficients of the tooling and of the material of the part to be brazed.
In practice, the brazing is carried out thanks to an assembly of metal tooling supporting the parts to be assembled. A filler metal having a melting temperature lower than that of the materials to be assembled, for example, of tin, copper, silver, aluminum, nickel or even precious metal alloys, is interposed between the two components to be assembled, then the assembly is heated at a temperature allowing the melting of the filler metal but not that of the components to be assembled. The liquefied filler metal wets the surfaces of the two components to be assembled. Under the effect of this rise in temperature, the expansion of the tooling provides a plating between the parts to be assembled.
The assembly is then cooled so as to solidify the filler metal between the two components to provide the connection.
Such a method is described for example in the international application WO 2003/092946, which targets a welding diffusion method.
As an indication, the mass of a tooling for a part of 1.2 meters of diameter and 0.6 meters of height is in the range of 1.4 tons, which requires in practice a cycle time, for heating and cooling the assembly, in the range of 13 hours.
If such a brazing method is absolutely applicable and advantageous to assemble together small-sized titanium parts, it does not hold true when the part to be manufactured is large-sized.
This is in particular the case when the part to be manufactured is a fixed inner structure of turbojet engine nacelle.
An aircraft is driven by several turbojet engines each housed in a nacelle. The nacelle has generally a tubular structure comprising an upstream air inlet of the turbojet engine, a middle section intended to surround a fan of the turbojet engine, a downstream section hosting the thrust reversal means and intended to surround the combustion chamber of the turbojet engine and, generally ended by an ejection nozzle located downstream of the turbojet engine.
The downstream section of a nacelle generally comprises an outer structure, called Outer Fixed Structure (OFS), which defines, with a concentric inner fixed structure, called Inner Fixed Structure (IFS), surrounding the engine structure itself downstream of the fan, an annular flow channel, channel also called flow path and intended to channel the cold air flow which circulates outside the engine.
An inner fixed structure of thrust reverser may have a diameter of 2.4 meters and a height of 2.7 meters. The metal tooling implemented to manufacture such a part by the brazing method as previously defined has a mass of 4.6 tons, which generates a very high thermal inertia making the brazing difficult. In addition, the cycle time for heating then for cooling the assembly with such a metal tooling is very long, in the range of 20 hours.
Furthermore, regardless of the size of the part to be manufactured, such a method is not adapted for the manufacture of a part whose the parts to be assembled have a very high coefficient of thermal expansion, for example the Inconel 625, material that can typically be used to manufacture the front portion of the ejection cone of hot gases from the turbojet engine.
It should in general be provided an ejection cone at the rear of an aircraft turbojet engine, in order to optimize the flow of the hot gases discharged by the turbojet engine on the one hand, and to absorb at least a portion of the noise generated by the interaction of these hot gases with the ambient air and with the cold air flow discharged by the turbojet engine fan on the other hand.
Such a conventional ejection cone 1 is shown in FIG. 1, in which the upstream and the downstream (relative to the flow direction of the exhaust gases of the turbojet engine) are respectively located on the left and on the right of the figure). This cone is intended to be positioned downstream of the turbine of the turbojet engine, concentrically to a shroud, or nozzle 3, itself fixed on the downstream edge of the combustion chamber of the turbojet engine. More specifically, the ejection cone 1 comprises, strictly speaking, a front portion of the cone 5 (often designated by “front plug”), of substantially cylindrical shape, and a rear portion of the cone 7 (often designated by “rear plug”), of conical shape.
These two portions of the ejection cone can typically be formed by metal alloy plates of Inconel 625 or titanium B21s type.
The front portion 5 can be particularly acoustic or monolithic stiffened. In the case where the front portion 5 is monolithic stiffened, this means that the structure is constituted of a unique plate reinforced with stiffeners. In the case where the front portion 5 is acoustic, it comprises at least one peripheral acoustic attenuation structure of sandwich type comprising at least one resonator, particularly of honeycomb type, covered with a perforated outer skin and with a full inner skin. The outer skin also constitutes an outer surface (plate) of the front portion of the cone 5.
The filler metals capable of being used for brazing parts made of Inconel have a relatively high cost.
To overcome this drawback, it is known from the prior art a brazing tooling by gas pressurization, tooling shown in FIG. 2, particularly allowing the brazing of the Inconel 625.
According to this prior art, the tooling comprises a central cylindrical cask 9 comprising at its periphery an outlet orifice (not shown), and a counter-form 11 having a shape substantially similar to the part to be manufactured. Typically, the counter-form is in two portions in order to be subsequently withdrawn when the part to be manufactured has a shape that cannot be removed from the mold.
An inner skin 13 of cylindrical shape, a honeycomb structure 14, then an outer skin 15 are placed between the central cask 9 and the counter-form 11.
Brazing sheets were interposed beforehand between each element constituting the inner skin/honeycomb structure/outer skin assembly.
The outer skin 15 is perforated and preformed (operation carried out first), that is to say of a shape substantially corresponding to the final shape which is desired to be given to the part, for example a domed shape as shown in FIG. 1.
The tooling further comprises at its upper and lower ends of the sealing flanges 17 and 19, intended to seal the tooling. These flanges are fixed to the central cask 9 by screwing.
The brazing method by gas pressurization consists of placing the tooling assembly in a vacuum furnace, and then introducing a gas, for example the argon, inside the central cask 9, diffusing through an outlet orifice of the central cask inside the inner skin 13.
An increasing in the furnace temperature causes the expansion of the gas, and therefore the deformation of the inner skin 13 and of the honeycomb structure, deforming until coming across the outer preformed skin 15, then until encountering the counter-form 11.
By increasing more the temperature, the brazing sheets melt and hold together the elements constituting the manufactured metal part during cooling.
The tooling assembly may then be dismounted by unscrewing the upper and lower sealing flanges, and by releasing the portions constituting the counter-form.
A major drawback of this type of tooling is that the sealing system is unreliable. The screws which allow the clamping of the flanges on the central cask 9 expand under the effect of pressure, which causes a decrease in the pressure on the sealing system causing leakages in the vacuum furnace.
In addition, the mounting and the dismounting of the sealing system are relatively complex, and the sealing time of the tooling is very long, which could go up to several hours.
Attempts were made to solve these problems by covering the screws of the flanges by a cover made of stainless steel, positioned above the tooling so as to allow a thermal protection of said screws.
However, despite the use of such protection covers, the screws of the sealing flanges expand again strongly, and this sealing system is not satisfactory.